The invention relates to a method for effecting precise modifications in predetermined surface layers of a workpiece by impacting those layers with cluster ions that are accelerated to a critical velocity and that are directed to impact a selected area of the workpiece at a preselected rate of impacts of cluster ions per square cm., or per unit area, per second. More particularly, the invention relates to a method for precisely modifying predetermined surface layers of a workpiece by either removing selected areas of such layers, or by compressing selected areas of such layers, or by enlarging the grain size of the crystalline structure of such layers, or by eroding or vaporizing preselected areas of an outer surface layer of a workpiece. In addition, the invention relates to a method of precisely modifying a workpiece of sheet or film material by making holes of predetermined, very small diameters through the workpiece. Such holes are made by simply impacting the workpiece with cluster ions, and without requiring the use of chemicals or other etching means to achieve the formation of the desired hole diameters.
A variety of methods and processes are known for effecting controlled very small modifications in many different kinds of workpieces. In general, such known processes and methods can be divided into two categories; namely, those that effect such modifications by chemical treatment of a workpiece, and those that effect physical modifications such as, for example micro-machining a workpiece with a tool, or with the application of a medium for effecting physical erosion of selected areas of the workpiece, or by vacuum deposition of material onto a surface of a workpiece.
Utility for the known methods and processes of effecting small modifications in workpieces has been found in; machining work surfaces, implanting layers of material onto a workpiece surface, changing the crystalline grain structure of a surface by chemically growing large crystals on the surface, making very fine filters by chemically etching bores that have been created in a workpiece by bombarding it with charged particles, as well as in numerous other applications of such methods. In many applications of known methods and processes, such as those used to manufacture micro-electronic devices, existing practical working limits are now restricting the attainment of any significantly higher packing densities for such devices. In order for the next generation of micro-electronic devices to supplant current state of the art silicon, gallium arsenide and similar inorganic p/n junction devices, extensive efforts are now being directed to develop methods for achieving greater packing densities in such devices by employing so-called monolayer assemblies of atoms or molecules in making the devices. As used in such a contest, and hereinafter, the so-called monolayer assemblies actually comprise systems that are a few, i.e., one to ten layers in thickness.
When chemical processes are developed to manufacture such so-called monolayer assemblies, the efforts tend to be directed at making mono-molecular layers of organic semiconducting polymers, which are called synthetic molecular organizates. When development efforts are directed at changing the physical orientations of such devices, there is a tendency to use more conventional semiconductor materials, such as gallium arsenide assembled in densely packed arrays of p/n junctions, which arrays make up so called super-lattices. The growing interest in using such atomic and molecular assemblies for manufacturing electronic devices is reflected in a recent issue of Scientific American, 249 (5), page 144 (November 1983), by G. H. Dohler, in which he describes an inorganic semiconductor method for making such devices. The solid state super-lattice devices thus described comprise crystals grown by depositing semiconductors in layers the thickness of which are measured in atoms. In an article in Chemical and Engineering News, p. 27 (May, 1983), by J. L. Fox, he describes the current interest exhibited by chemists in the possibilities of making molecular scale devices that rely on organic polymers and charge transfer complexes. More recently it has been proposed to use mono-molecular film cation/anion membranes in advanced electronic and electrochemical information processing and energy control applications, such as in micro-electronic circuit chips, and fuel cells.
Two common characteristics of all such research and development efforts is their aim to develop or discover methods that can (1) afford more precision control of modifications effected in a workpiece, and (2) achieve smaller and smaller precise modifications in a workpiece. Thus, improved future methods should afford the desired denser packing capabilities for microelectronic devices, as well as enabling both more predictable and finer control of micro-machining operations on workpieces, and in the manufacture of filters having finer filtering capabilities than heretofore known.
Because the present invention relates to a method that uses accelerated cluster ions to precisely modify a workpiece, rather than being a method that employs chemical reactions, per se, to achieve such a modification, the closest known prior art methods discussed below, as being of comparable interest, do not list chemical methods. Thus, methods are known in the prior art, whereby; (1) single atoms or molecules are accelerated to either implant them deeply into a workpiece, or to bombard a workpiece and achieve secondary emission of particles from it, (2) large cluster ions are used to bombard a workpiece and effect sputtering of large masses of material from it, and (3) low velocity cluster ions randomly mixed with neutral clusters, are used to clean surfaces and to deposit films on an outer surface of a workpiece. One example of the type of uses explored with the first of such prior art methods is described in an articled entitled, "Production and Use of Nuclear Tracks: Imprinting Structure on Solids", by B. E. Fischer, et al., in Review of Modern Physics, Vol. 56, No. 4, October 1983 (pp. 907-948). That article explains how nuclear or atomic tracks create damaged zones in a workpiece along the paths made by rapidly moving accelerated ions that impact a workpiece. It points out that, most frequently such random track arrays are now employed to induce global property changes of the solid volume or surface of a workpiece.
A representative publication explaining known uses of very large cluster ions, is an article entitled, "Micrometeorite Simulation Studies on Metal Targets", by H. Dietzel, et al., which appeared in Journal of Geophysical Research, Mar. 10, 1972, pp. 1375-1394. That paper reports on experiments using microparticles accelerated at from 0.2 to 40 kilometers per second and having a mass ranging in size from greater than 3.times.10.sup.-10 grams and less than 3.times.10.sup.-13 grams. Such particles were used to bombard thin plates of aluminum, copper, cadmium, tin and several other metals to form craters in a polished surface thereof.
A representative publication explaining known uses of low velocity cluster ions, that are randomly mixed with neutral clusters, is an article entitled, "Film Formation by Ionized-Cluster Beam Deposition", by T. Takagi, et al., which appeared in Conference Series No. 38, Low-energy Ion Beams, 1977, (Sept. 5-8, 1977), published by The Institute of Physics, Bristol and London, UK.
In all such types of known prior art methods, i.e. those for accelerating either single atoms or molecules, those for accelerating so called micrometeoroid cluster ions, and those for accelerating low velocity cluster ions, it is obviously necessary to employ a suitable accelerator. Many types of such accelerators are known and a discussion of several different hypervelocity accelerators is given in a book entitled, "High Velocity Impact Phenomena", by R. Kinslow, published in 1970 by Academic Press. Chapter 1 entitled, "Hypervelocity Accelerators", explains that such accelerators basically can be categorized as either gun accelerators or explosive accelerators. The method of the present invention, in its disclosed preferred embodiments as discussed herein, utilizes a gun type accelerator for accelerating cluster ions, as is more fully explained below. A further example of a type of accelerator apparatus that is useful in accelerating cluster ions is described in an article by two of the inventors named in the present application, i.e. by R. J. Beuhler and L. Friedman. The article is entitled, "Threshhold Studies of Secondary Electron Emission Induced by Macro-ion Impact on Solid Surfaces". It was published during 1980 in Nuclear Instruments and Methods. pp. 309-315. That article explains how water cluster ions in the m/e range 3.7.times. 10.sup.2 to 6.0.times.10.sup.4 were formed in an ion source consisting of a 0.625 mm diameter copper wire placed approximately 1.2 centimeters away from a first expansion aperture of about 0.15 mm diameter, of a supersonic molecular beam source. A positive 6 kv potential was applied to the wire, while a mixture of water vapor and nitrogen was passed concentrically around it toward the above-noted first expansion aperture. It was reported that the mass of the resultant water cluster ions could be conveniently varied over an extended range of sizes by selectively varying the flow rate and/or temperature of the nitrogen mixing gas. Subsequently, cluster ion acceleration was accomplished in a 34-stage, 93 centimeter long, acceleration column. The electric field in the acceleration column was approximately 10 kilovolts/cm.
Another detailed description of a suitable apparatus for the production and acceleration of cluster ions is presented in copending U.S. patent application No. 452,362, by Messrs. L. Friedman and R. J. Beuhler, which application was filed Dec. 22, 1982.
All of the types of known prior art methods for utilizing accelerated particles, i.e., those using atoms or molecules, or those using dust-particle-size micrometeoroid ions, or those using low velocity cluster ions randomly mixed with neutral ion clusters, have not been found capable of precisely modifying a predetermined surface layer of a workpiece, or of forming precision-diameter holes through a workpiece, or of performing in the optimum manner of the present invention the other advantageous precision functions that can be achieved with its practice. A particular disadvantage of those prior art methods, relative to the method of the present invention, is that they inherently produce a more random distribution of energy in a workpiece than is characteristic of the carefully controlled diffusion of energy in a precise area of a workpiece, by appropriate application of the method disclosed herein. A major advantage of the novel method of the present invention is that it provides a means for concentrating precisely predetermined amounts of energy in a narrow preselected area and depth of a workpiece, thereby to effect a desirably precise, small modification in that area. In that sense, the advantage and utility of the method of the present invention, relative to known prior art methods, is somewhat analogous to the dramatic advantages realized from the coherent energy and the precisely controlled, focused power of a laser beam, compared to the dispersed energy achievable from a normal light beam.